Answer
$\theta=100.893^o$.
Work Step by Step
Let $u=(0,1,-1),\quad v=(1,2,3), \quad \langle u, v\rangle=u \cdot v $.
The angle $\theta$ between $u$ and $v$ is given by the formula $$\cos \theta=\frac{ \langle u, v\rangle}{\|u\| \cdot\|v\|}=\frac{0+2-3}{\sqrt{ 0+1+1}\sqrt{1+4+9}}=\frac{-1}{\sqrt{28}}$$
that is $\theta=100.893^o$.
